Mesosaurus lived in freshwater lakes and ponds.
environment in which mesosaurs lived was believed to have been very shallow water that was drying and shrinking, like what is happening today to the Aral Sea or Urmia Lake
Evidence say that they were alive about 300 million years ago and their species went extinct about 260 million years ago.
Now coming back to the question what kind of surrounding in which mesosaurus lived
Well they used to live in freshwater and ponds and sources say that they were among the first reptiles to live in and around water, and they